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Abstract

Coherent anti-Stokes Raman scattering (CARS) microscopy is demonstrated to be a promising optical method for the characterization of polymer films with film thickness varying between 180 nm to 4300 nm. In case of PMMA films with a thickness of few hundreds of nanometers, the observed CARS signal was mainly associated with the interference effect of large nonresonant CARS field from glass substrate and the weak resonant field of PMMA. The dependence of resonant CARS intensity of PMMA film on film thickness is in good agreement with the theoretical prediction on a CARS field. The current work offers potential possibilities of noninvasive thickness measurement of polymeric thin film of thickness less than 180 nm by multiplex CARS microscopy without depth-profiling.

Figures (6)

PMMA film thickness as a function of repeated number of coatings. The inset shows cross sectional profile of the ablated surface of PMMA thin film with 1500 nm in thickness. The thickness was determined by measuring the height difference between the intact surface of PMMA and the bare glass substrate.

Multiplex CARS (a) and cw-Raman spectral features (b) of PMMA thin films coated on a slide glass substrate. Raman peaks at 2842 cm-1, 2932 cm-1, 2952 cm-1, 3000 cm-1, and 3058 cm-1 are in good agreement with reported values [15]. (c-g) are the selected CARS spectral features of PMMA films with a thickness of 180 nm, 715 nm, 1310 nm, 2070 nm, and 4300 nm, respectively. The experimental observation was denoted with black circle. The red solid lines are the best spectra resulted from the fitting procedure of the observed spectra with Eq (3-6) (see text).

CARS intensity as a function of the thickness of the PMMA film. The experimental observation was denoted with black circle. Blue circles are the resonant CARS signal from PMMA film resulted from the fitting procedure of the observed spectra with Eq (3-6) (see text). Red solid line represents the dependence of resonant CARS signal resulted from theoretical consideration of CARS field scattered from PMMA disk. For comparison, the line with a slope of two was also displayed by green dotted line.

Multiplex CARS intensity profile (blue solid circles) along axial direction for about 870 nm thick film of PMMA coated on slide glass (a). The intensity profile of resonant CARS signal from PMMA (black solid circles) was fitted by Gaussian function. Full width at half maximum was found to be ca. 2.1 um (red solid line). (b-f) represents specificity of depth profile at several depth positions. The experimental observation was denoted with black circle. The red (blue, green, magenta) solid lines are the best results from total (PMMA, cross term, only glass) spectral fitting using Eq. (3).